Method for separating molybdenum from tungsten by electrolysis from a fused electrolyte bath



United States Patent 3,075,900 METHOD FOR SEPARATING MOLYBDENUM FROM TUNGSTEN BY ELECTROLYSIS FROM A FUSED ELECTROLYTE BATH John B. Zadra and John M. Gomes, Reno, Nev., assignors to the United States of America as represented by the Secretary of the Interior No Drawing. Filed Nov. 25, 1960, Ser. No. 71,835 11 Claims. (Cl. 20464) (Granted under Title 35, U.S. Code (1952), see. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to the separation of molybdenum during the fused bath electrolysis of tungsten minerals to produce tungsten.

It has been known heretofore that tungsten may be produced by the fused-bath electrolysis of tungsten oxide, or tungsten ores. Such processes are described in FINK et al. U.S. Patents 2,463,367 and'2,554,527. However, one difiiculty encountered is that with ores containing molybdenum, under the reaction conditions employed, both molybdenum and tungsten are recovered.

Scheelite (CaW 4), which is the tungsten mineral predominantly mined in the United States, is almost invariably accompanied by molybdenum, which may amount to over 3 percent. In certain applications molybdenum is an objectionable contaminant and must be removed to achieve high-purity metal. The technique employed heretofore is to remove the moylbdenum .from the ore by a relatively complex chemical purification method, such as shown in the United States patent to Smith, 2,339,888. After removal of the molybdenum, the remaining tungsten compound is subject to any of the known methods for recovery of tungsten, such as electrolysis, orhydrogen reduction.

It has now been discovered that tungsten ores containing molybdenum may be subject to electrolysis under controlled conditions whereby the molybdenum will deposit first, and then the tungsten. At a low current density the molybdenum deposits out first from the electrolytic bath. Therefore, the two metals may be deposited on separate electrodes by first employing a low current density to remove the molybdenum, and then a higher current density to deposit the tungsten. In some instances, agitating the electrolyte is advantageous in that local depletion of molybdenum from the immediate vicinity of the electrode is avoided. The tungsten may thus be recovered in a substantially pure form, relatively free from molybdenum.

It is the primary object of this invention to provide a method for the electrolytic separation of molybdenum from tungsten, from tungsten ores containing molybdenum, or from compositions containing both elements.

A further object is to provide an electrolytic method for the preparation of relatively pure tungsten from its ores which have a molybdenum content.

A further object is to provide an electrolytic method for the recovery of molybdenum from tungsten ores having a small percentage of molybdenum.

A further object is to provide an electrolytic method for separating molybdenum and tungsten by varying the current density.

Another object of the invention is to provide an agitated bath of electrolyte during the deposition of the molybdenum metal.

Other objects of the invention will become apparent upon reference to the specification.

In accordance with the present invention, 'a scheelite ore containing molybdenum in an appreciable amount,

is dissolved in a molten solvent to form an electrolyte, and placed in an electrolytic cell. Current is supplied between the electrodes, a suitable current density being maintained, to deposit out the molybdenum. After the molybdenum content is reduced to the degree desired, a new cathode is inserted and the current density is increased. Tungsten having a reduced molybdenum content is then deposited thereon.

If desired, the tungsten may be deposited on the same cathode as the molybdenum. By operating the process to remove substantially all the molybdenum from the bath of electrolyte, tungsten having as little as .001 to .005 by weight molybdenum may be obtained.

Scheelite is almost invariably accompanied by molybdenum in varying amounts, usually from about 0.5% to over 3%. This and other tungstenic ores or materials containing molybdenum may be employed in the process. The molybdenum content of the tungstenic material can be quite high without affecting the operability of the method.

Any of the solvent baths employed heretofore for the electrolytic recovery of tungsten may be employed. Thus, alkali phosphate mixtures of sodium pyrophosphate, sodium metaphosphate and sodium chloride; mixtures of sodium tetraborate, sodium pyrophosphate, and sodium ohloride; sodium tetraborate and sodium chloride are satisfactory. When scheelite is the tungstenic material, sodium tetraborate used alone as the solvent results in the formation of insoluble calcium borate, which greatly increases the viscosity of the electrolyte and interferes with electrolysis. The addition of sodium chloride prevents this, and results in the formation of a homogeneous electrolyte. Besides acting as a fluxing agent for the CaO, the sodium chloride serves alsoto increase the bath fluidity.

Satisfactory mixtures for use With scheelite are the following compositions, which are cited by way of examples, and are not to be construed as being limited thereto:

(a) 7 moles sodium pyrophosphate, 3 moles sodium metaphosphate, and 14 moles sodium chloride. Sodium tetraborate may replace a portion of the sodium pyrophosphate, and the amount of sodium chloride may be increased to 25 moles. In percentages, the composition of sodiumpyrophosphate, sodium metaphosphate and sodium chloride,.respectively, range approximately from 60%, 10%, 30%, to 50%, 10%, and 40%.

(b) 46 mole percent sodium tetraborate and 54- mole percent sodium ohloride, or about and 25% respectively, by weight.

Instead of the sodium compounds, the corresponding salts of any member of the alkali metal group of elements may be employed, and other halogens may be substituted, for chlorine.

During the electrolysis the bath temperature is maintained at a temperature of between 900 C. and 1100 C. In general, the preferred range is between about 950 C. and 1100 C. ,In the case of scheelite, variations in the optimum temperatures for different molybdenum contents and solvent bath occur within this range.

Current density from depositing molybdenum may range from about 2 to about 30 a./dm. with a preferred rang of about 7 a./dm. to about 30 a./.dm. For tungsten, the current density may be within the broad range of about 20 to about 200 a./dm. withthe preferred range being from about 40 to about a./dm. Current. density at the anode is not critical. At intermediate ranges there, would be both metals deposited, and for'close separation it is preferable to select a current density at the low end of the range for the deposition of molybdenum, and then a high current density for the deposition of tungsten.

The amount of tungstenic material in the electrolyte bath may vary between broad ranges. From about 1.4 to about 5 parts of solvent to one part of tungstenic material by weight may be employed. However, the preferred range is from 2 to 4 parts of solvent to one part of tungstenic material.

Any suitable material, such as graphite, tungsten, or cold-rolled steel, may be used for the cathode. In the examples given below, the cell consisted of a graphite crucible as the anode, with a graphite rod electrode as the cathode.

The following examples illustrate specific embodiments of the invention:

EXAMPLE 1 A solvent mixture consisting of sodium pyrophosphate, 255 grams (61.1%); sodium met-aphosphate, 42 grams (10.1%); sodium chloride, 120 grams (28.8%), was weighed out. It is apparent that the percentages of ingredients are substantially 60%, and 30%, respectively. To this was added 176 grams of scheelite having a molybdenum content of 2.96% and a tungsten content of 54.20%. The ratio of solvent mixture to scheelite was 2.4:1. The solids were then mixed and placed in a graphite electroylsis cell consisting of a 2% inch 1.d. graphite crucible as the anode and a 1 inch graphite rod as the cathode. The crucible anode was placed in a furnace and the temperature raised to melt the salts and form the electrolyte, and thereafter a constant electrolyte temperature of 950 C. was maintained.

Current density on the cathode was held at 10 a./dm. for the removal of molybdenum during the first six intervals of 7 ampere hours, and then increased to 47 a./dm. for the deposition of tungsten. At the end of each 7 ampere-hour interval, the cathode was removed and a new one inserted. The metal deposited on each cathode was collected and analyzed. The results are given in the following table:

Table 1Separati0n of Molybdenum From Tungsten at Various Ampere-Hours, Stationary Cathode From the table, it is seen that at the end of 7 amperehours, 50 percent of the molybdenum was extracted, but only 1.5% of the tungsten. At 28 ampere-hours, 82.1 percent of the molybdenum was extracted and 14 percent of the tungsten. The extraction of only 85% of the molybdenum was due apparently to the loss of molybdenum-bearing electrolyte adhering to the cathode when exchanged at each 7 ampere-hour interval.

EXAMPLE 2 To determine the possible effect of local depletion of metal in the vicinity of the cathode and to maintain a homogeneous electrolyte, a rotating 1 inch diameter graphite cathode which stirred the electrolyte was substituted for the stationary cathode of Example 1. The technique employed was the same except that the current density was reduced to 7 a./dm. for the first three intervals for the removal of molybdenum and then increased for the last three intervals for the removal of tungsten. The results obtained are shown in Table 2 below:

Table 2-Separati0n of Molybdenum From Tungsten at Various Amper-Hours, Rotating Cathode Extraction, Cumulative, Current Analysis, percent percent Ampere-hours density, percent aJdm. M0

M0 W Mo W It is seen from the table that a 7.5 ampere-hours 52.1 percent of the molybdenum and only 0.1 percent of the tungsten were extracted; at 21.25 ampere-hours, 98 percent of the molybdenum and 5.1 percent of the tungsten were extracted.

Stirring the electrolyte apparently assists in preventing metal depletion around the vicinity of the cathode and is at least a contributing factor in the extremely high separation of the molybdenum from tungsten.

The spent electrolyte contains insoluble compounds and impurities comprising about 33 percent of the total. The remaining 67 percent comprise useful reagents, which are water soluble and can be regenerated by crystallization.

It will be apparent that various modifications in the embodiments shown can be made within the scope of the invention as defined in the appended claims.

We claim:

1. A method for producing substantially pure tungsten metal from a scheelite ore containing molybdenum, which comprises mixing said ore with a solvent composition consisting of sodium chloride and at least one member of the class consisting of sodium pyrophosphate, sodium metaphosphate, and sodium tetraborate, the ratio of solvent to scheelite being from about 1.4:]. to about 5:1 by weight, heating the mixture of scheelite and solvent to fusion to dissolve the scheelite and thereby form an electrolyte, maintaining the electrolyte at a temperature of from about 900 C. to about 1100 C., electrolyzing said electrolyte at a current density of between about 2 a./dm. to about 30 a./dm. for a time sufllcient to deposit out the molybdenum on a cathode, removing said cathode and inserting a fresh cathode into the electrolyte and then increasing the current density to above that employed in the deposition of molybdenum and within the range of from about 20 a./dm. to about 200 a./clm. to deposit out the tungsten.

2. A method for producing substantially pure tungsten metal from a scheelite ore containing molybdenum, which comprises mixing said ore with a solvent composition consisting of sodium chloride and at least one member of the class consisting of sodium pyrophosphate, sodium metaphosphate, and sodium tetraborate, the ratio of solvent to scheelite being from about 2:1 to about 5:1 by weight, heating the mixture of scheelite and solvent to fusion to dissolve the scheelite and thereby form an electrolyte, maintaining the electrolyte at a temperature of from about 900 C. to about 1100 C., electrolyzing said electrolyte at a current density of between about 7 a./dm. to about 30 a./dm. for a time suincient to deposit out the molybdenum on a cathode, removing said cathode and inserting a fresh cathode into the electrolyte and then increasing the current density to above that employed in the deposition of molybdenum and within the range of from about 40 a./dm. to about 100 a./dm. to deposit out the tungsten.

3. The method of claim 2, wherein the solvent composition consists of sodium pyrophosphate, sodium metaphosphate, and sodium chloride.

4. The method of claim 2, wherein the solvent composition consists of sodium tetraborate and sodium chloride.

5. The method of claim 1, wherein the electrolyte is agitated during electrolysis.

6. The method of claim 1, wherein the electrolyte is agitated during electrolysis.

7. A method for producing substantially pure tungsten metal from a scheelite ore containing molybdenum which comprises mixing said ore with a solvent composition consisting of sodium pyrophosphate, sodium metaphosphate, and sodium chloride, the ratio of solvent to scheelite being about 2:1 to 4:1 by weight, heating the mixture of scheelite and solvent to fusion to dissolve the scheelite and thereby form an electrolyte, maintaining the electrolyte at a temperature of about 950 C., electro lyzing said electrolyte at a current density of about a./dm. for a time suflicient to deposit out the molybdenum on a cathode, removing said cathode and deposited molybdenum, inserting a fresh cathode into the electrolyte and then increasing the current density to about 47 a./dm. to deposit out the tungsten.

8. The method of claim 7, wherein the solvent means consists of about 50% to 60% sodium pyrophosphate, about 10% sodium metaphosphate, and about 30% to 40% sodium chloride, all percentages being by weight, and wherein the ratio of solvent to scheelite is about 2.4 to 1 by weight.

9. A method for producing substantially pure tungsten metal from scheelite ore containing molybdenum which comprises mixing said ore with a solvent composition consisting of sodium pyrophosphate, sodium metaphosphate, and sodium chloride, the ratio of solvent to scheelite being about 2:1 to 4:1 by weight, heating the mixture of scheelite and solvent means to dissolve the scheelite and thereby form an electrolyte, maintaining the electrolyte at a temperature of about 930 C., agitating said electrolyte, electrolyzing said electrolyte at a 'current density of about 7 a./dm. until substantially all the molybdenum present has been deposited on a cathode, removing said cathode and deposited molybdenum, inserting a fresh cathode and increasing the current density between about 15 and about a./dm. to deposit out the tungsten.

10. The method of claim 9, wherein the solvent composition consists of about 50% to sodium pyrophosphate, about 10% sodium metaphosphate and about 30% to 40% sodium chloride, all percentages being by weight, and wherein the ratio of solvent to scheelite is about 2.4:1 by Weight.

'11. An electrolyte suitable for the recovery of tungsten, comprising a fused mixture of about sodium tetraborate and 25% sodium chloride, said mixture having scheelite dissolved therein in a ratio of fused salts to scheelite of about 2:1 to about 4:1.

References Cited in the tile of this patent UNITED STATES PATENTS 873,648 Aiken Dec. 10, 1907 1,202,534 Keyes et a1. Oct. 24, 1916 2,554,527 Fink et al May 29, 1951 2,624,702 Merre Jan. 6, 1953 2,956,936 Huber et a1. Oct. 18, 1960 FOREIGN PATENTS 203,614 Australia Sept. 13, 1956 OTHER REFERENCES Leo et al.: Transactions of the Electra Chemical Society, vol. 66, pages 461-469 (1934).

Electrowinning Tungsten and Associated Molybdenum From Scheelite (Zadra et al.), Bureau of Mines, Report of Investigations 5554 (1959), pages 18-23. 

1. A METHOD FOR PRODUCING SUBSTANTIALLY PURE TUNGSTEN METAL FROM A SCHEELITE ORE CONTAINING MOLYBDENUM, WHICH COMPRISES MIXING SAID ORE WITH A SOLVENT COMPOSITION CONSISTING OF SODIUM CHLORIDE AND AT LEAST ONE MEMBER OF THE CLASS CONSISTNG OF SOIDUM PYROPHOSPHATE, SODIUM METAPHOSPHATE, AND SODIUM TETRABORATE, THE RATIO OF SOLVENT TO SCHEELITE BEING FROM ABOUT 1.4:1 TO ABOUT 5:1 BY WEIGHT, HEATING THE MIXTURE OF SCHEELITE AND SOLVENT TO FUSION TO DISSOLVE THE SCHEELLITE AND THEREBY FORM AN ELECTROLYTE MAINTAINING THE ELECTROLYTE AT A TEMPERATURE OF FROM ABOUT 900*C. TO ABOUT 1100*C., ELECTROLYZING SAID ELECTROLYTE AT A CURRENT DENSITY OF BETWEEN ABOUT 2 A./DM.2 TO AOUT 30 A./DM2 FOR A TIME SUFFICIENT TO DEPOSIT OUT THE MOLYBDENUM ON A CATHODE, REMOVING SAID CATHODE AND INSERTING A FRESH CATHODE INTO THE ELECTROLYTE AND THEN INCREASING THE CURRENT DENSITY TO ABOVE THAT EMPLOYED IN THE DEPOSITION OF MOLYBDENUM AND WITHIN THE RANGE OF FROM ABOUT 20 A./DM2 TO ABOUT 200 A./DM.2 TO DEPOSIT OUT THE TUNGSTEN. 